1. Field of the Invention
The present invention relates to admixtures for slurries and in particular cementitious slurry compositions.
2. Description of the Related Art
As is well known in the art, most cementitious compositions are laid down or used in a slurry form. Increasing difficulty and expense in obtaining high quality aggregate for use in such cementitious material such as concrete has forced manufacturers to resort to low grade materials such as crushed stone, marine sand and even recycled crushed concrete obtained from demolitions or old structures. This leads to problems with the concrete such as a higher water demand, bleeding (where, as the slurry settles, water migrates to the surface), lower workability and pumpability.
In the past, these problems have been overcome by the addition of certain additives to the cementitious composition. These plasticisers, sometimes known as water reducers, dispersion agents or super plasticisers, act to increase the workability and validity of the slurry for a given quantity of water. Examples include lignosulphonates, naphthalene sulphonate-formaldehyde condensates.
Typically, these water reducers are added at around 0.3% by weight of cement and provide between 8 and 12% reduction in the water cement ratio, depending upon the addition procedure. Additions of up to 1% by cement provide up to 35% reduction in the water to cement ratio. In high performance concrete application, eg ultra high strength concrete, it is common to overdose in plasticiser/water reducer, (or combinations thereof) to obtain further water reduction of up to 50%. However, at such dosage levels detrimental effects are produced, eg setting times increased and compressive strength of a cementitious mixture reduced.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.